The invention relates to an illumination unit comprising an electrodeless low-pressure discharge lamp,
which illumination unit is provided with
a lamp vessel enclosing a discharge space which contains an ionizable filling,
a holder for the lamp vessel,
a coil for generating a high-frequency magnetic field for maintaining an electric discharge in the discharge space,
a high-frequency supply for the coil, and
ignition aids for promoting an initiation of the discharge,
which coil is provided with a primary and a secondary winding, the primary winding having a first and a second end which are connected to a first and a second contact, respectively, of the supply, while in a nominal operational state of the illumination unit the first contact supplies a high-frequency voltage and the second contact supplies a reference voltage free from high-frequency variations, in which state a voltage gradient arises in the primary winding in a direction from the first to the second end, and in the secondary winding a voltage gradient is generated in a direction from a first to a second end which is opposed to that from the first to the second end in the primary winding, while high-frequency variations of the voltage averaged over the joint surface area of the primary and the secondary winding are comparatively small in relation to high-frequency variations of the voltage averaged over the surface area of the primary winding.
The invention also relates to an electrodeless low-pressure discharge lamp suitable for use in the illumination unit, and also to a holder and to a supply unit suitable for use in such an illumination unit.
Such an illumination unit is known from EP 0.332.263 A1. In the known illumination unit, the supply is accommodated in a housing fastened to the lamp vessel. A first contact of the supply delivers a voltage which varies with high frequency, i.e. with a frequency above 20 kHz. In fact, the frequency is approximately 3 MHz. The supply has a second contact which supplies a reference voltage free from high-frequency variations. The reference voltage is, for example, equal to earth potential, but may alternatively be any other voltage free from high-frequency variations. This asymmetrical supply may be comparatively simple in comparison with a symmetrical supply, i.e. a supply in which the voltages at the contacts are in mutually opposed faces.
The lamp vessel has a luminescent layer at its inside and is provided with a filling containing mercury. The coil has a primary winding of 13 turns and a secondary winding of 14.5 turns. The turns in this known illumination unit all have the same winding direction and extend over a length of 30 mm around a core of soft-magnetic material with a length of 50 mm. The secondary winding has a first free end. An opposed, second end is connected to the second contact of the supply. During operation, a voltage is generated in the secondary winding which has a gradient from the reference voltage at the second end to a voltage at the first end which has approximately the same value, but with opposite sign, as that of the first contact. High-frequency variations in the voltage averaged over the coil surface therefore have an amplitude which is comparatively small compared with high-frequency variations of the voltage averaged over the surface area of the primary winding. The interference caused by the illumination unit is accordingly comparatively small compared with that in the case of an illumination unit with a primary winding only.
With the primary and the secondary winding having the same winding direction, the voltage gradients in these windings have the same direction. A voltage gradient accordingly occurs in the secondary winding which is opposed to that from the first to the second end of the primary winding in a direction opposed to that from the first to the second end of the primary winding. The direction from the first to the second end of the primary winding accordingly is opposed to that from the first to the second end of the primary winding.
If the winding direction of the secondary winding is chosen to be opposed to that of the primary winding, the voltage gradients in these windings are of opposite directions. In that case the direction from the first to the second end of the secondary winding is equal to that from the first to the second end of the primary winding.
The voltage gradient in the primary winding and that in the secondary winding jointly result in an electric field in the discharge space. This field is capable of igniting the lamp at a sufficiently high amplitude of the voltage delivered by the supply. The known illumination unit has an electrical conductor at the first end of each of the windings. The conductors carry a voltage which differs comparatively strongly from the reference voltage. Unlike the windings, the conductors are at a comparatively great distance from one another. These ignition aids accordingly prevent the electric field lines from concentrating around the windings. An electric field is thus generated in the discharge space which is strong enough for igniting the lamp already at a comparatively low voltage across the primary winding.
This measure, however, has the disadvantage that charged particles in the discharge space, such as mercury ions, react with the wall material during operation owing to the comparatively strong electric field. The wall suffers a comparatively strong blackening thereby and mercury becomes unavailable for the discharge. The useful life of the illumination unit is adversely affected by this effect.
The invention has for its object to provide a measure in an illumination unit of the kind mentioned in the opening paragraph which also results in a comparatively low ignition voltage of the lamp, but which renders it possible to avoid the above disadvantage.
According to the invention, the illumination unit is for this purpose characterized in that, after switching-on, the ignition aids achieve that the nominal operational state is preceded by an ignition state in which the second end of the secondary winding is disconnected from the second contact of the supply unit.
It is surprising that the secondary winding need not hamper lamp ignition in spite of the fact that in the ignition state a voltage gradient is generated in the secondary winding whose value corresponds to that obtaining during the nominal operational state. Since the second end of the secondary winding is disconnected from the second contact of the supply in the ignition state, the interval over which the voltage has its gradient differs from that during the nominal operational state. Contributions to the electric field resulting from high-frequency voltage differences between the windings and earth do not cancel one another out in the ignition state. The inventors have found that a considerably lower ignition voltage can suffice under these circumstances compared with the case in which the second end of the secondary winding is connected to the second contact of the supply. Since the ignition state lasts much shorter than the nominal operational state, the measure renders it possible to avoid the disadvantage mentioned above.
In an illumination unit according to the invention, the secondary winding may be permanently connected with one end to the second contact of the supply. The illumination unit is then brought into the ignition state in that a bipolar switch interchanges the connections of the ends of the primary winding to the supply contacts. This switching of the connections reverses the voltage gradient in the primary, and thus also in the secondary winding. The voltage interval traversed in the primary winding remains the same. In the secondary winding, however, the voltage interval traversed during the ignition state differs from that during the nominal operational state. The voltage is equal to the reference voltage at the permanently connected end. Since the voltage gradient is reversed, the voltage changes towards the other end to a value opposed to that which it would have during the nominal operational state. Electric fields caused by voltage variations in the secondary winding thus reinforce those which are derived from the primary winding, so that the lamp will ignite readily. The switching of the connections causes the ends to interchange their functions. In the ignition state, accordingly, the second end is an end which is disconnected from the second contact of the supply.
An attractive embodiment of the illumination unit according to the invention is characterized in that the ignition aids between the second end of the secondary winding and the second contact of the supply comprise a first switch which is open in the ignition state and closed in the nominal operational state. The switch is, for example, a semiconductor switch. A comparatively great reduction in the ignition voltage can thus be realised by comparatively limited means.
A favourable modification of the above embodiment is characterized in that the secondary winding is decoupled in the ignition state. A single switch can suffice in this way. The average voltage of the secondary winding cannot change because no charge can flow to or from the secondary winding in the decoupled state. High-frequency variations in the average voltage of the primary winding are not compensated by those in the secondary winding as a result.
In a further favourable modification of the above embodiment of the invention, the ignition aids in addition comprise a second switch, which is closed in the ignition state and open in the nominal operational state, between the first contact of the supply and the first end of the secondary winding. In this modification, it is realised by simple means that the secondary winding and the primary winding together generate an electric field in the ignition state which is more effective than would have been possible with the primary winding alone, given the voltage at the first supply contact.
The ignition state may have a predetermined duration, for example, because a timer switch is used which causes the illumination unit to enter the nominal operational state, for example, 100 ms after switching-on.
Preferably, the transition from the ignition state to the nominal operational state is dependent on a lamp parameter. It is achieved in a simple manner thereby that the ignition state does not end before the lamp has ignited. The beginning of the nominal operational state depends, for example, on the power consumed by the lamp.
An attractive embodiment of the illumination unit according to the invention is characterized in that said lamp parameter is a temperature. This may be, for example, the temperature in a location at the surface of the lamp vessel. The transition from the ignition state to the nominal operational state is realised, for example, by means of one or several bimetal switches arranged in the vicinity of the lamp vessel. Such switches may be comparatively inexpensive.
A very attractive embodiment is characterized in that said lamp parameter is light generated by the lamp. The nominal operational state can then commence immediately after the lamp has ignited.
In an embodiment of the illumination unit according to the invention, the supply unit is accommodated in a holder fastened to the lamp vessel.
Alternatively, the supply unit may be accommodated in a separate housing, the electrodeless lamp being connected to the supply unit by means of a cable. The invention accordingly also applies to an electrodeless low-pressure discharge lamp suitable for use in an illumination unit as described with reference to one of the above embodiments of the invention, which lamp is provided with the lamp vessel, the holder, the coil, and the ignition aids.
The lamp vessel may be detachably coupled to the holder. If so desired, it may then be replaced by a new lamp vessel, for example, one provided with a luminescent layer which luminesces at a different colour temperature. A holder according to the invention suitable for use in an illumination unit as described above is provided with the coil and with the ignition aids, while the lamp vessel can be coupled to the holder.
The ignition aids for the lamp may form a supply device in conjunction with the supply unit. A supply device according to the invention suitable for use in an illumination unit as described above with reference to one of the embodiments is provided with the high-frequency supply and with the ignition aids.
An embodiment of the supply device according to the invention is characterized in that the supply device is in addition provided with the coil.